Apparatus for preparing purified terephthalic acid

ABSTRACT

A method and apparatus for purifying crude terephthalic acid from a liquid dispersion thereof also containing impurities selected from unreacted starting materials, solvents, products of side reactions and/or other undesired materials is provided. The method comprises the steps of filtering the dispersion to form a crude terephthalic acid filter cake, dissolving the filter cake in a selective crystallization solvent at an elevated temperature to form a solution, crystallizing purified terephthalic acid from the solution in the crystallization solvent by reducing the pressure and temperature of the solution, and separating the crystallized purified terephthalic acid from the solution. According to the invention, the selective crystallization solvent is non-aqueous, non-corrosive and essentially non-reactive with terephthalic acid. Preferably, the selective crystallization solvent is N-methyl pyrrolidone. The method and apparatus produces purified terephthalic acid having a purity desired for use in forming polyester resin and other products at an economically attractive rate and at operating conditions of reduced severity which require a lower capital investment and simplified processing.

This application is a divisional of application Ser. No. 08/760,890,filed Dec. 6, 1996, U.S. Pat. No. 6,013,835, which is acontinuation-in-part of application Ser. No. 08/477,898, filed Jun. 7,1995, U.S. Pat. No. 5,767,311.

This application is a continuation-in-part of co-pending applicationSer. No. 08/477,898, filed Jun. 7, 1995, entitled Method and Apparatusfor Preparing Purified Terephthalic Acid, which is assigned to the sameassignee as this application, and the totality of the disclosure ofwhich is hereby incorporated by reference for all purposes.

The present invention relates to a method and apparatus for preparingpurified terephthalic acid. It also relates to methods and apparatusesfor purifying crude terephthalic acid to produce a purified terephthalicacid product which is a useful starting material for producing polyesterresin, which is in turn useful for the production of fibers, film,plastic bottles, and polyester resin structures, often reinforced byother materials such as glass fiber.

BACKGROUND OF THE INVENTION

Purified terephthalic acid (PTA) is a starting material for theformation of polyester resin, which is, in turn, used to make manymaterials of commerce having a variety of utilities. Purifiedterephthalic acid is formed from "crude" terephthalic acidconventionally by a number of purification methods, often with the aidof catalysts. The methods for purifying crude terephthalic acidheretofore available are not completely satisfactory either from anengineering standpoint, or from an economic standpoint, yet the purityof the purified terephthalic acid is an important determinant of thesatisfactoriness of the processes by which the polyester resin isformed.

A number of reaction systems are known for forming crude terephthalicacid from a variety of starting materials. The purification aspects ofthe present invention may be used with substantially any of thesereaction systems, but in accordance with the invention it is preferredthat a reaction system involving the oxidation of paraxylene (p-xylene)be employed, and the use of such a synthesis system forms a part of thepresent invention.

The problems of existing and prior systems for producing purifiedterephthalic acid center around the difficulties in running the reactionsystems to produce good yields of crude terephthalic acid economically,compounded by the difficulties of refining the crude terephthalic acidto eliminate impurities and unwanted components to produce purifiedterephthalic acid of a quality suitable as a starting material forproducing polyester. Concomitant problems in prior systems include thehigh capital investment required for PTA plants, the severity ofoperating conditions of prior processes, both for the production ofcrude terephthalic acid, and for its purification, and the need forhandling catalyst systems and reaction solvents, as well as reactionbyproducts in a way such that environmental problems are minimized, andloss of material is also controlled.

One factor of importance in the production of purified terephthalic acidis the formation of crystals having a size and shape giving them goodhandling characteristics, washability and filterability.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided a method andapparatus for producing purified terephthalic acid. In one aspect, themethod includes the production of crude terephthalic acid by theoxidation of p-xylene. The oxidation step produces not only terephthalicacid, but by side reactions p-toluic acid and 4-carboxybenzaldehyde(4-CBA). The product produced in the oxidation step is a liquiddispersion containing unreacted starting materials, solvents, if anyhave been used, the products of side reactions, particularly those justmentioned, and other materials which are not desired in the sought-forpurified terephthalic acid. The oxidation step of the present inventionis so conducted that the conversion to crude terephthalic acid should beat least about 30% by weight per pass of p-xylene.

In further accordance with the invention, the crude terephthalic acidfrom the oxidizer is first grossly separated from the other materialsfrom the oxidizer and then it is re-dissolved in a selectivecrystallization solvent and, optionally, one or more additional solventsof the invention discussed below. The re-dissolved crude terephthalicacid is then crystallized out of the selective crystallization solventand additional solvents of the invention in one or, preferably, twocrystallization stages. Provision is made to separate out thecrystallized and progressively purified terephthalic acid from thesolvents of the invention, and the filter cake of purified terephthalicacid ultimately obtained is washed with other solvents of the inventionand dried or, alternatively, dried (e.g., using a vacuum dryer), sent toa soaker to remove residual solvent, and ultimately filtered and driedfor storage or for further processing.

Also in accordance with the present invention, improvements in thecrystallization processes just outlined are provided which producelarger more globular crystals that are thought to contain little or nosalt of the kind which may tend to form when the selectivecrystallization solvent(s) is an organic base. The larger non-saltcrystals have the advantage that they resist destruction by waterrinsing and are otherwise easier to recover solvent from, as well asbeing easier to rinse for removal of residual impurities.

The improvements in the crystallization processes comprise flashingsolvent from the crystallizing acid by reducing the pressure on it,preferably both prior to and during the cooling of the saturated acidsolution. It is further preferred to reduce the pressure progressivelyto lower levels, either in a batch or continuous flow crystallizer, andthis may be arranged to be performed stepwise or continuously. Stillfurther, heat may be added to the crystallizing acid during theapplication of reduced pressure to increase the rate and quantity ofsolvent removal, care being taken, however, to avoid materiallyincreasing the temperature of the crystallizing acid to causeredissolution of the acid and consequent waste of energy.

As was mentioned above, in accordance with the invention,crystallization may be performed in multiple stages; when this form ofthe invention is used, it is preferred that some or all of thecrystallization improvement techniques just discussed be utilized in thesecond or last stage, although the techniques may also be used toadvantage in the first stage.

Further in accordance with the invention, co-solvents may be used forpurifying terephthalic acid by flash crystallization. A co-solventhaving a lower boiling point than the solvent can be used to reduce theflashing temperature for crystallization and hence the dissolutiontemperature. With a lower flashing temperature, crystallization can becarried out under a lower degree of vacuum.

The co-solvents include water, C₁ to C₅ alcohols, such as methanol orethanol, C₅ to C₁₀ hydrocarbons, such as p-xylene, and C₁ to C₁₀ organicacids, such as formic acid or acetic acid, etc. It is thus possible toinclude about 1 to about 50% inert solvents having boiling pointsranging from 25 to 200° C. as the co-solvents.

The invention also contemplates that steps are included to reclaim andrecycle the solvents of the invention at each stage of crystallizationand washing, including recycle of some of the recovered materials to theoxidizer. Steps are also taken to closely control the delivery of anyobjectionable materials to the environment.

In an important aspect, the present invention is based on severaldiscoveries relating to solvents which are effective to bring about thepurification of crude terephthalic acid through crystallization andseparation steps. These discoveries may be summarized in several ways asfollows.

The selective crystallization solvents useful in the practice of thepresent invention include those in which (a) the impurities desired tobe separated from terephthalic acid to purify it are relatively moresoluble in the solvent than is the terephthalic acid at substantiallyevery temperature within the desired range of temperatures at which thesolvent containing terephthalic acid is to be handled, and (b) theterephthalic acid is more soluble at an elevated temperature and lesssoluble at a lower or reduced temperature. It is to be understood thatthe term "selective crystallization solvent" is intended to meansolvents useful in the selective crystallization of terephthalic acid asdescribed above and as described in greater detail below and as shown inFIGS. 1 and 2.

In this connection it should be noted that U.S. Pat. No. 3,465,035mentions that certain organic solvents (pyridine, dimethyl sulfoxides,dimethyl foramide, and the like) have been used to purify terephthalicacid, but that they suffer from being unstable in air and easily formaddition products with terephthalic acid. This same patent, along withseveral others, also teaches the use of acetic acid and water aspurification solvents for terephthalic acid. By contrast, the selectivecrystallization solvents according to the present invention are (a)non-aqueous, (b) non-corrosive, and (c) essentially non-reactive withterephthalic acid and do not include those prior practices justdescribed. Specifically, water, acetic (and other alkyl) acid, and theabove-mentioned organic solvents are excluded from the selectivecrystallization solvents which are contemplated by the presentinvention.

In accordance with the invention, the primary preferred selectivecrystallization solvents are N-methyl pyrrolidone (NMP) and N,N-dimethylacetamide (DMAC), for the several reasons discussed below, and for theirsuperior performance. U.S. Pat. No. 2,949,483, dated Aug. 16, 1960 toHam, discloses NMP used to crystallize terephthalic acid, but does notuse the same dissolution temperature range as in the present invention.Nor does it suggest flash crystallization or its advantageous results.Tr. Vses. Nauch.-Issled. Proekt. Inst. Monomerov (1970), 2(2), 26-32;From: Ref. Zh., Khim. 1971, Abstr. No. 1N166; V. N. Kulakov, et al.;entitled "Purification of Aromatic Dicarboxylic Acids Obtained byLiquid-Phase Oxidation of Dialkyl Derivatives of Aromatic Hydrocarbons,"very briefly mentions NMP as a solvent, but says nothing aboutdissolution temperatures or flash crystallization.

N-methyl pyrrolidone (NMP) and N,N-dimethyl acetamide (DMAC) are thepreferred selective crystallization solvents for the practice of theinvention. These solvents are non-aqueous, thermally stable, non-toxic(environmentally safe), non-corrosive, and commercially available. NMPis the most preferred selective crystallization solvent for the practiceof the present invention, because its solubility versus temperaturecurve for terephthalic acid slopes upwardly and to the right, whichmeans that terephthalic acid can be dissolved in it at elevatedtemperatures, and precipitated or crystallized from it at lowertemperatures.

Although NMP is the most preferred selective crystallization solvent, itis to be understood that DMAC exhibits similar desirable characteristicsand that, in accordance with the present invention, other preferredselective crystallization solvents for purification of crudeterephthalic acid can be selected from various polar organic solventsincluding, but not intended to be limited to, N-alkyl-2-pyrrolidone(such as N-ethyl pyrrolidone), N-mercaptoalkyl-2-pyrrolidone (such asN-mercaptoethyl-2-pyrrolidone), N-alkyl-2-thiopyrrolidone (such asN-methyl-2-thiopyrrolidone), and N-hydroxyalkyl-2-pyrrolidone (such asN-hydroxyethyl-2-pyrrolidone), N-ethyl pyrrolidone, N-mercaptoethylpyrrolidone, N-methyl thiopyrrolidone, N-hydroxyethyl pyrrolidone,1,5-dimethyl pyrrolidone, N-methyl piperidone, N-methyl caprolactam,N-formyl morpholine, morpholine, N,N-dimethyl formamide, N,N-dimethylacetamide, and N-formyl piperidine, and the like, and mixtures thereof.Still other selective crystallization solvents contemplated by thepresent invention include, but are not intended to be limited to,sulfolane, methyl sulfolane, the sulfones, the morpholines (such as,morpholine and N-formyl morpholine), the carbitols, C¹ to C¹² alcohols,the ethers, the amines, the amides, and the esters, and the like, andmixtures thereof.

It is preferred that the desired selective crystallization solvent beused in a multi-stage crystallization process in combination with one ormore additional solvents, preferably two such additional solvents,particularly where the crude terephthalic acid is less than about 98%pure. Preferably, a wash solvent, such as, but not intended to belimited to, water, p-xylene, acetone, methyl ethyl ketone (MEK) ormethanol, and the like, is used in the washing of the initial filtercake obtained from the first separation of crude terephthalic acid fromother materials issuing from the oxidizer. In addition, a displacementsolvent having a low boiling point, such as, but not intended to belimited to, water, methanol, acetone, MEK, and the like, may be used.Preferably, water is used as the displacement solvent in associationwith the third filter following the second crystallization stage in thepreferred process. The desired displacement solvent displaces theselective crystallization solvent from the resulting filter cake,whereby substantially only the displacement solvent is present duringthe soaking process. The soaking process is preferred to eliminate anypossible residual solvent trapped in the TA crystals before the productis subjected to the final filtration and drying steps.

As described above, NMP and DMAC are the preferred selectivecrystallization solvents for the practice of the invention. They arenon-aqueous, thermally stable, non-toxic (environmentally safe),non-corrosive, and commercially available. NMP is the preferredselective crystallization solvent for the practice of the presentinvention, because, among other things, its solubility versustemperature curve for terephthalic acid slopes upwardly and to theright, which means that terephthalic acid can be dissolved in it atelevated temperatures, and precipitated or crystallized from it at lowertemperatures. However, the solubility versus temperature curve forterephthalic acid is of a much milder slope than the solubility curvesin NMP for other materials sought to be separated from crudeterephthalic acid, such as benzoic acid, 4-carboxybenzaldehyde (4-CBA),and p-toluic acid. As a consequence, when crude terephthalic acid,containing or associated with unreacted starting materials, solvents (ifany), and products of side reactions, such as those mentioned above, orother undesired materials, is dissolved in NMP or DMAC at an elevatedtemperature, substantially all the materials are dissolved or at leasthighly dispersed. Then upon removal of heat and pressure and subsequentcooling of the NMP or DMAC solution of such dissolved materials, thepure terephthalic acid preferentially crystallizes out, while the othermore soluble materials which may be regarded as impurities for thepresent purposes remain in solution in NMP or DMAC. A separation is thuseffected between purified terephthalic acid and its associatedimpurities. NMP or DMAC may be stripped of the impurities in areclaiming column and recycled into the process, while the impuritiesmay be recycled to the oxidizer step or otherwise disposed of.

From the foregoing, it can be seen that in accordance with one aspect ofthe present invention, a method is provided for producing purifiedterephthalic acid from crude terephthalic acid in which the crudeterephthalic acid is dissolved in a desired crystallization solvent atan elevated temperature to form a solution and further, in which apurified terephthalic acid is crystallized from that solution at areduced pressure and temperature.

In accordance with another aspect of the invention, a method andapparatus are provided for purifying crude terephthalic acid from aliquid dispersion thereof also containing unreacted starting materials,solvents, products of side reactions, and/or other undesired materialsin which the crude terephthalic acid is filtered from that dispersion topartially separate it from the other materials contained therein byfiltration to produce a crude terephthalic acid filter cake, and thendissolving that filter cake in a desired selective crystallizationsolvent at an elevated temperature to form a solution. Purifiedterephthalic acid is crystallized from that solution by reducing thepressure and temperature thereof and is separated from the solventfollowing crystallization.

In accordance with still another aspect of the invention, a method andapparatus are provided for producing purified terephthalic acid fromcrude terephthalic acid by dissolving the crude terephthalic acid in adesired selective crystallization solvent at an elevated temperature toform a first solution. First stage purified terephthalic acid iscrystallized from that first solution at a reduced temperature, andpreferably at a reduced pressure also. The first stage purifiedterephthalic acid is separated from the solvent solution of otherimpurities and redissolved in the desired selective crystallizationsolvent at an elevated temperature to form a second solution. Thissecond solution is crystallized at a reduced pressure and temperature toform a second stage purified terephthalic acid and the second stagepurified terephthalic acid is separated from the second solution.

In accordance with yet another aspect of the invention, crudeterephthalic acid is synthesized by contacting paraxylene with oxygen inan oxidizer reaction. The crude terephthalic acid is withdrawn from theoxidizer and separated grossly from the side products of the reaction,and unreacted starting materials. The separated crude terephthalic acidis then dissolved in a desired selective crystallization solvent at anelevated temperature and crystallized from it as purified terephthalicacid at a reduced pressure and temperature. More than one stage ofdissolving in a desired selective crystallization solvent at an elevatedtemperature followed by crystallization at a reduced pressure andtemperature, with accompanying separation and washing of thecrystallized purified terephthalic acid, may be performed.

From the foregoing, it can be seen that an object of the presentinvention is to provide an improved method and apparatus for producingpurified terephthalic acid of a purity desired for use in formingpolyester resin and other products, at an economically attractive rate,and at operating conditions of reduced severity which require a lowercapital investment and simplified processing. The manner in which theseand other objects of the invention are attained may be learned byconsideration of the detailed description of the invention whichfollows, together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the method and apparatus of the presentinvention may be obtained by reference to the following DetailedDescription when taken in conjunction with the accompanying Drawingswherein:

FIGS. 1 and 2 are plots of solubility versus temperature curves forterephthalic acid and for impurities or side reaction products commonlyassociated with crude terephthalic acid in NMP and DMAC, respectively;and

FIG. 3 is a simplified elevational diagram of a crystallizer which maybe used in the practice of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS I. Process Description

The present invention relates to the development of a new PTAmanufacturing technology. Compared to the current widely used PTAtechnology, this technology provides a substantially lower capitalinvestment in new PTA plant construction, as well as lower costs ofplant operation. It also provides means for current DMT plants toco-produce PTA, to strengthen their competitiveness against newer PTAplants.

Process Summary

The success of this process is based on the development of a lowpressure, low temperature, non-aqueous, highly selective crystallizationtechnology. The crystallization technology can purify the crudeterephthalic acid (TA) with purity as low as from between about 70%(from the oxidizer) and about 98+% in the first-stage crystallizer, andabout 99.99+% in the second-stage crystallizer. This allows the TAoxidizer to be operated at much lower severity than those of widely usedprior art processes. No acetic acid (as solvent/diluent) orbromine-catalyst initiator is needed in the oxidizer in accordance withthe present invention. The selective crystallization solvent used in thecrystallization process is non-aqueous, thermally stable, non-toxic(environmentally safe), non-corrosive, and commercially available.

When carrying out the method according to the present invention,employing NMP or DMAC as the selective crystallization solvent, thepresent inventors have demonstrated TA purity levels of up to 99.9+wt %after a first crystallization process, and up to 99.99+wt % after asecond crystallization process. In particular, Table 1 illustrates therecovery of 99.95 wt % pure TA after the first crystallization processand 99.997 wt % pure TA after the second crystallization process, fromcrude TA (89.89 wt % TA).

                  TABLE 1                                                         ______________________________________                                        Crystallization   1st Crystallization                                                                       2nd                                             ______________________________________                                        (a) Weight of TA:     56.34 grams 31.81 grams                                 (b) Weight of Crystallization                                                                       400.02 grams                                                                              248.38 grams                                    Solvent:                                                                  (c) Saturation Temperature:                                                                         60° C.                                           (d) Crystallization Temperature:                                                                    15° C. (one hour)                                (1) Crude TA Product Composition:                                             Benzoic p-Toluic 4-CBA      TA      Others                                    0.39 wt %                                                                             4.49 wt %                                                                              2.49 WT %  89.89 WT %                                                                            274 WT %                                  (2) First Crystallization Product                                             35 ppm    143 ppm    359 ppm    99.95 wt %                                                                    Not                                                                           Detected                                      (3) Second Crystallization Product                                            <20 ppm   <20 ppm    <10 ppm    99.997 + wt %                                 ______________________________________                                    

Table 2 illustrates the recovery of 99.90wt % pure TA after the firstcrystallization process and 99.9933wt % pure TA after the secondcrystallization process from crude TA (89.89wt % TA) by increasing boththe saturation temperature and the crystallization temperature.

                  TABLE 2                                                         ______________________________________                                        Crystallization   1st Crystallization                                                                       2nd                                             ______________________________________                                        (a) Weight of TA:     138.08 grams                                                                              70.15 grams                                 (b) Weight of Crystallization                                                                       685.30 grams                                                                              247.46 grams                                    Solvent:                                                                  (c) Saturation Temperature:                                                                         110° C.                                                                            105° C.                              (d) Crystallization Temperature:                                                                    40° C.                                                                             40° C.                               (1) Crude TA Product Composition:                                             Benzoic p-Toluic  4-CBA     TA       Others                                   0.39 wt %                                                                             4.49 wt % 2.49 wt % 89.89 wt %                                                                             2.74 wt %                                (2) First Crystallization Product (Recovery: 56.5 wt %)                       Benzoic p-Toluic  4-CBA     TA        Others                                  28 ppm  367 ppm   390 ppm   99.90 wt %                                                                              229 ppm                                 (3) Second Crystallization Product (Recovery: 47.5 wt %)                      <10 ppm <19 ppm   25 ppm    99.9933 wt %                                                                            13 ppm                                  ______________________________________                                    

Table 3 illustrates the recovery of 99.9960wt % pure TA (singlecrystallization process) from crude TA (98.99wt % TA). In addition, eachof benzoic, p-Toluic, 4-CBA, MMT and other impurities were at less than10 ppm.

                  TABLE 3                                                         ______________________________________                                        (a)     Weight of TA:      152.67 grams                                       (b)     Weight of Crystallization                                                                        786.19 grams                                               Solvent:                                                              (c)     Saturation Temperature:                                                                          100° C.                                     (d)     Crystallization Temperature:                                                                     40° C.                                      (1) Crude TA Product Composition:                                             Benzoic                                                                              p-Toluic 4-CBA    TA      MMT    Others                                <10 ppm                                                                              <10 ppm  18 ppm   98.99 wt %                                                                            303 ppm                                                                              0.98 wt %                             (2) Crystallization Product (Recovery: 50.2 wt %)                             <10 ppm                                                                              <10 ppm  <10 ppm  >99.9960                                                                              <10 ppm                                                                              <10 ppm                                                        wt %                                                 ______________________________________                                    

Table 4 illustrates the recovery of 99.63wt % pure TA (singlecrystallization process) from crude TA (83.91 wt % TA) on a large scalebasis.

                  TABLE 4                                                         ______________________________________                                        (a)     Weight of TA:      1760 grams                                         (b)     Weight of Crystallization                                                                        6162 grams                                                 Solvent:                                                              (c)     Saturation Temperature:                                                                          160° C.                                     (d)     Crystallization Temperature:                                                                     50° C.                                      (1) Crude TA Feed Product Composition:                                        Benzoic p-Toluic  4-CBA     TA       Others                                   1.03 wt %                                                                             4.79 wt % 5.03 wt % 83.91 wt %                                                                             5.24 wt %                                (2) Crystallization Product (Recovery: 24.3 wt %)                             38 ppm  852 ppm   0.23 wt % 99.63 wt %                                                                             500 ppm                                  ______________________________________                                    

Table 5 illustrates the recovery of 99.92 wt % pure TA (singlecrystallization process) from crude TA (79.79 wt % TA) on a large scalebasis.

                  TABLE 5                                                         ______________________________________                                        (a)     Weight of TA:      1700 grams                                         (b)     Weight of Crystallization                                                                        5928 grams                                                 Solvent:                                                              (c)     Saturation Temperature:                                                                          160° C.                                     (d)     Crystallization Temperature:                                                                     45° C.                                      (1) Crude TA Feed Product Composition:                                        Benzoic p-Toluic  4-CBA     TA       Others                                   1.59 wt %                                                                             5.19 wt % 7.61 wt % 79.79 wt %                                                                             5.81 wt %                                (2) Crystallization Product (Recovery: 31.5 wt %)                             10 ppm  203 ppm   446 ppm   99.92 wt %                                                                             184 ppm                                  ______________________________________                                    

Table 6 illustrates the recovery of 99.15 wt % pure TA (singlecrystallization process) from crude TA (83.90 wt % TA) on a large scalebasis at a higher saturation temperature of 190° C.

                  TABLE 6                                                         ______________________________________                                        (a)     Weight of TA:      1965 grams                                         (b)     Weight of Crystallization                                                                        5684 grams                                                 Solvent:                                                              (c)     Saturation Temperature:                                                                          190° C.                                     (d)     Crystallization Temperature:                                                                     40° C.                                      (1) Crude TA Feed Product Composition:                                        Benzoic p-Toluic  4-CBA     TA       Others                                   1.23 wt %                                                                             5.25 wt % 6.34 wt % 83.90 wt %                                                                             3.28 wt %                                (2) Crystallization Product (Recovery: 48.9 wt %)                             --      0.14 wt % 0.61 wt % 99.15 wt %                                                                             0.1 wt %                                 ______________________________________                                    

Table 7 illustrates the recovery of 99.9915 wt % pure TA from crude TA(98.50 wt % TA) on a large scale basis. The supersaturation of thecrystallization mixture resulted in the formation of substantiallylarger TA crystals than those crystals resulting from the processessummarized above. As would be understood by one skilled in the art, thesizes of TA crystals are an important consideration with respect toseparation thereof from solvents and impurities.

                  TABLE 7                                                         ______________________________________                                        (a)     Weight of TA:      2333 grams                                         (b)     Weight of Crystallization                                                                        5698 grams                                                 Solvent:                                                              (c)     Saturation Temperature:                                                                          160° C.                                     (d)     Crystallization Temperature:                                                                     45° C.                                      (1) Crude TA Feed Product Composition:                                        Benzoic p-Toluic  4-CBA     TA       Others                                   198 ppm 0.15 wt % 1.23 wt % 98.50 wt %                                                                             989 ppm                                  (2) Crystallization Product (Recovery: 69.7 wt %)                             <10 ppm 26 ppm    38 ppm    99.9915 wt %                                                                           11 ppm                                   ______________________________________                                    

Table 8 demonstrates the recovery of 99.45 wt % pure TPA (singlecrystallization process) from crude TPA (82.92 wt % TPA), usingN,N-dimethyl acetamide (DMAC) as the crystallization solvent. The 4-CBAcontent was reduced from 6 wt % to 3276 ppm. The range of the operatingtemperature was very moderate (from 45 to 120° C.).

                  TABLE 8                                                         ______________________________________                                        Purifying "TPA" with N,N-Dimethyl Acetamide by Crystallization                1.  N,N-dimethylacetamide used:                                                                         1,000.0 grams                                           Crude TPA used:       291.5 grams                                             N,N-dimethylacetamide for cake wash:                                                                800 ml                                                  Purified TPA recovered:                                                                             135.6 grams (not                                                              including losses due to                                                       solids handling                                                               and sampling)                                       Sample   Benzoic  PTA     4-CBA TPA    Unknown                                ______________________________________                                        Crude TPA                                                                              5.25     6.01    4.59  82.92  1.23                                   (wt %)                                                                        Purified TPA                                                                           689      3276    1519   99.45*                                                                              13                                     (ppm)                                                                         ______________________________________                                        *weight percent                                                               2. Method:                                                                    (a) The mixture was heated to 120° C. in an agitated and jacketed          flask                                                                         to dissolve the solids, and the mixture was held at the temperature           for one hour.                                                             (b) The mixture was then cooled to 45° C. in one hour.                 (c) The cooled slurry was then filtered in a separatory funnel under              vacuum to separate the mother liquor from the cake.                       (d) The cake was washed once in the separatory funnel with the solvent            to remove the residual mother liquor in the cake. The wash was                carried out at room temperature.                                          (e) The wet solid was soaked over night with D.I. water at room                   temperature and then washed three times with D.I. water in a                  separatory funnel.                                                        (f) The solids were dried over night at 180° C.                        ______________________________________                                    

As has been discussed, important aspects of this invention are relatedto the discovery of methods to crystallize terephthalic acid (TA) fromorganic solution where the solvent tends to form an organic salt withTA. The salt is normally formed from cooling the solution of an organicsolvent or a mixture of organic solvents, which solution is saturatedwith TA at higher temperatures. However, crystal structure of the saltis destroyed when it is washed with water or other solvents to removethe solvent in the crystal. The washed crystals become very fine powderswhich are very difficult to filter and wash in order to remove theimpurities in the trapped mother liquor and the residual solvent.

According to this invention, the solution of an organic solvent (ormixture of organic solvents) saturated with TA and impurities such as4-carboxybenzaldehyde (4-CBA), p-toluic acid, etc., is fed to acrystallizer maintained at a lower pressure (or under vacuum) to allowthe solvent (or solvent mixture) to flash instantaneously in acontinuous or batch manner. Then, the solids (nuclei) generated fromsolvent flashing are allowed to grow for a certain period of time at thereduced pressure and temperature. It is desirable to subject thesaturated solution to a number of solvent flash operations in the samecrystallizer or in several crystallizers connected in series, each at adifferent reduced pressure (or vacuum), to generate higher TA recoveryand larger TA crystals. It has been found, surprisingly, that thestructure of the crystals produced from this method is not adverselyaffected by washing with water or other solvents which have significantsolubility of the crystallization solvent (or mixture of solvents) or byvacuum drying the crystals to remove solvent. Consequently, it appearsthat there was no salt formation or at least the salt formation wasminimized so that washing with water or other solvent which can dissolvethe crystallization solvent or vacuum drying did not change the size andshape of the TA crystals.

As previously mentioned, organic solvents useful in this inventioninclude, but are not limited to, N-methyl pyrrolidone (NMP), N-ethylpyrrolidone, N-mercaptoethyl pyrrolidone, N-methyl thiopyrrolidone,N-hydroxyethyl pyrrolidone, 1,5-dimethyl pyrrolidone, N-methylpiperidone, N-methyl caprolactam, N-formyl morpholine, morpholine,N,N-dimethyl formamide, N,N-dimethyl acetamide, N-formyl piperidine,N-alkyl-2-pyrrolidone (such as N-ethyl pyrrolidone),N-mercaptoalkyl-2-pyrrolidone (such as N-mercaptoethyl-2-pyrrolidone),N-alkyl-2-thiopyrrolidone (such as N-methyl-2-thiopyrrolidone), andN-hydroxyalkyl-2-pyrrolidone (such as N-hydroxyethyl-2-pyrrolidone).

In order to remove the residual solvent trapped in the crystals from thefinal TA product, the washed TA crystals are preferably fed to a hightemperature soaker where water is used to partially or completelydissolve the TA crystals.

The following examples illustrate the principles and features of theinvention.

EXAMPLE 1 Cooling Crystallization

9761 g of NMP was added to a jacketed crystallizer provided withagitation together with 3028 g of TA. This mixture was heated to 180° C.under atmospheric pressure until all of the TA was dissolved.

The mixture was then subjected to surface cooling by circulating acooling medium through the jacket until a temperature of45° C. wasreached. Then after 15 minutes, the slurry was filtered to separate thesolids from the mother liquor, and the cake was washed with roomtemperature pure NMP to displace all the mother liquor from the cake.

A sample was taken from the cake for observation under a microscope. Thecrystals had a bar-like shape and a size in the range of 120-150microns.

In order to remove the solvent from the cake, the cake had to be washedwith water or other suitable solvents which have high solubility of thesolvent. Hot water at 80° C. was used to wash the cake. However, thebar-like crystals in the cake were completely destroyed by water andchanged into fine powders which looked more like precipitates thancrystals produced by a crystallization process. These fine precipitatesare extremely difficult to wash and handle and the residual solventremoval is complicated.

EXAMPLE 2 Flashing Crystallization

The same sample preparation of NMP and TA as in the previous example wasused, except that the mixture was also, prior to the cooling step,subject to a flashing removal of solvent by reducing the pressure fromatmospheric to 125 mmHg of vacuum. In this way, some solvent wasvaporized out and condensed through a cooler so the temperature of themixture dropped from 180° C. to 147° C. The amount of solvent flashedout created a super-saturation condition so the TA dissolved in NMPcrystallizes into the solid phase.

Although the flashing step is done instantaneously, crystallization ofTA requires some time to take place, so the mixture was kept agitatedfor 30 minutes to form the nuclei and permit them to grow, thus forminga slurry. The slurry was filtered to separate the solids from the liquidphase, washed with pure NMP at room temperature and observed under amicroscope. The crystal shape was globular instead of bar-like, as itwas when using the previous cooling crystallization method, and veryuniform in size but smaller--about 40-60 microns.

Then the cake was washed with hot water at 80° C. and, surprisingly, theglobular-like crystals were not affected by water washing (their shapeand size were not changed). These globular-like crystals have a veryhigh filtration rate and effectively rinsing them is much easier.

EXAMPLE 3a Crystal Growth

To promote crystal growth, the experiment, as in the preceding example,was repeated except that 6174 g of NMP and 1952 g of terephthalic acidwere used.

Also, the flash pressure was 120 mmHg instead of 125 mmHg and thetemperature was 145° C. Then, the mixture was flashed a second time at40 mmHg, as described in the preceding example, and the temperaturedropped to 110° C. Thus, more terephthalic acid crystallized. Thecrystal shape was globular-like and the size was increased to 60-80microns.

EXAMPLE 3b

The experiment as in Example 3a was repeated except that 7490 g of NMPand 2325 g of terephthalic acid were used. Also, a different pressureprofile was followed and two more flashes were added:

    ______________________________________                                        first flash:      150 mmHg @ 154° C.                                   second flash:     80 mmHg @ 135° C.                                    third flash:      40 mmHg @ 117° C.                                    ______________________________________                                    

Observation under a microscope showed that the crystal shape wasglobular and the size improved significantly. The final sample containedcrystals in the range of 120-150 microns.

EXAMPLES 4a AND 4b Flash/Vaporizing Crystallization

The experiment as in Example 3b was repeated except that the temperatureof the hot oil circulation through the jacket was kept 5-10° C. abovethe crystallizer temperature in a way that some vaporization of thesolvent occurred at the same time of the flashing. This procedureresulted in more solvent flashed/vaporized and a lower temperatureprofile which increases the recovery of the crystals:

    ______________________________________                                        FLASH                                                                         No.   EXAMPLE 3b   EXAMPLE 4a   EXAMPLE 4b                                    ______________________________________                                        First 154° C.                                                                             155° C.                                                                             145° C.                                      150 mmHg     150 mmHg     150 mmHg                                            755 ml of solvent                                                                          1328 ml of solvent                                                                         1660 ml of solvent                                  removed by flashing                                                                        removed by flashing                                                                        removed by flashing                           Second                                                                              135° C.                                                                             135° C.                                                                             130° C.                                      80 mmHg      80 mmHg      80 mmHg                                             696 ml of solvent                                                                          473 ml of solvent                                                                          580 ml of solvent                                   removed by flashing                                                                        removed by flashing                                                                        removed by flashing                           Third 117° C.                                                                             110° C.                                                                             115° C.                                      40 mmHg      40 mmHg      40 mmHg                                             248 ml of solvent                                                                          110 ml of solvent                                                                          340 ml of solvent                                   removed by flashing                                                                        removed by flashing                                                                        removed by flashing                           Fourth                                                                              101° C.                                                                             90° C.                                                                              95° C.                                       20 mmHg      20 mmHg      20 mmHg                                             135 ml of solvent                                                                          155 ml of solvent                                                                          430 ml of solvent                                   removed by flashing                                                                        removed by flashing                                                                        removed by flashing                           ______________________________________                                    

When observed under a microscope, the crystals looked globular-like inshape as described for Example 2 above.

EXAMPLE 5

In this example, the 4-CBA rejection characteristics of the flashcrystallization method was compared with that of crystallization bycooling alone.

Flash Crystallization

The crystallizer was charged with 31 g TA/100 g solvent. 4-CBA was addedto start with a concentration based on solids of around 2%. The mixturewas heated to 185° C. and agitated until most of the crystals dissolved.Some crystals may not have dissolved and these became seeds for crystalgrowth. The oil bath was set to 155° C. The first vacuum (150 mmHg) waspulled to remove around 15-20% of the liquid in about 15 minutes. Next,the flash vacuum was pulled to 80 mmHg and 6-8% of the remaining liquidwas removed within 5 minutes. In the third flash, 6-8% of the solventwas removed with a vacuum of 40 mmHg requiring about 6-7 minutes. In thefourth flash, 12% of the solvent was removed with a vacuum of 20 mmHgrequiring about 10-15 minutes. Then the mother liquor was cooled to 50°C. as quickly as possible, taking about 30 minutes. The crystals werethen removed from the flask and filtered using a Buchner funnel and sidearm flask. About 200 g of 50° C. solvent was then poured over to washthe crystals. The crystals were then put in a pressure filter and driedby passing nitrogen for 30 minutes at 40 psi. The final crystals wereanalyzed for 4-CBA content, giving a result of 500 ppm.

Cooling Crystallization

The crystallizer was charged with 31 g TA/100 g solvent. 4-CBA was addedto start with a concentration based on solids of 2%. The mixture washeated to 185° C. and agitated until most of the crystals dissolved.Some crystals may not have dissolved and these became seeds for crystalgrowth. Cooling of the mix was started to crystallize the TA from thesolution. The cooling rate was 2° C./min to a final temperature of 50°C. The crystals were then removed from the flask and filtered using aBuchner funnel and side arm flask. About 200 g of 50° C. solvent wasthen poured over to wash the crystals. The crystals were then put in apressure filter and dried by passing nitrogen for 30 minutes at 40 psi.These final crystals were analyzed for 4-CBA content, giving a result ofabout 500 ppm.

The experiments show that the flash and cooling crystallizationprocesses have substantially the same rejection capability for 4-CBA.

According to the invention, a preferred embodiment of the process isdivided into five sections:

(1) Oxidation Section:

In this section, p-xylene is oxidized according to the following mainreactions:

    ______________________________________                                        (a)    p-xylene + oxygen -> terephthalic acid + water                         (b)    p-xylene + oxygen -> p-toluic acid + water                             (c)    p-xylene + oxygen -> 4-carboxybenzaldehyde (4-CBA) +                          water                                                                  ______________________________________                                    

The oxidizer residence time is approximately five hours. Since theoxidizer effluent will contain up to about 30% TA, mixing in theoxidizer is very important in order to maintain the yield andselectivity, and to prevent fouling and blockages. The initial mixing ofthe feed streams may be achieved in a static mixer (outside of theoxidizer). Further mixing may be provided by an air sparger and byexternal circulation. Depending on the thoroughness of the p-xylenewashing step at the filter (discussed below), the terephthalic acid (TA)in the solid can vary from between about 55% and about 90+%.

(2) Crystallization Section:

(A) First Crystallization

After filtration, the solids from the oxidizer effluent are mixed withthe mother liquor and the solvent wash liquid from the second-stagecrystallizer and with additional crystallization solvent. The mixedslurry is dissolved in a slurry tank at a predetermined temperature,preferably at from between about 140° C. and about 200° C.. Thesaturated solution is transferred to a holding tank to remove p-xylenethrough evaporation. The saturated solution is then fed to a first-stagebatch crystallizer to recover purified TA by flash evaporation ofsolvent at reduced pressure and/or cooling. After the crystallizationstep, the crystallizer content is then dropped to a product holding tankand is pumped continuously to a filter (or centrifuge) to collect thesolids to be recrystallized in the second-stage crystallizer for furtherpurification.

(B) Second Crystallization

The solids generated from the first crystallizer filter are redissolvedin a feed dissolver with the crystallization solvent for thesecond-stage crystallizer at a predetermined condition, such as at atemperature of from between about 140° C. and about 200° C. Thesaturated solution is pumped to the second-stage crystallizer forcrystal growth and recovery, again, by flash evaporation of solvent atreduced pressure and/or cooling. Then, the crystallizer content isdropped to a holding tank for filtration before being sent to thesoaker. In the filtration step, the solid (cake) is first washed by thecrystallization solvent to displace mother liquor remaining in the cake.The solid is then washed by a low-boiling solvent to displace thecrystallization solvent in the cake and subsequently dried to remove thefinal liquid from the PTA product. The crystallization solventalternatively can be displaced by drying the solid using a vacuum dryerand subjecting the cake to a soaking process. The soaking processcomprises partially or completely dissolving the TA in a solvent,crystallizing the product in water at a high temperature and highpressure to remove residual solvent trapped in the crystals, andrecrystallizing, filtering and drying the TA cake.

(3) Mother Liquor/Solvent Recovery Section:

The mother liquor from the first crystallizer filter is transferred to asolvent recovery column to recover the crystallization solvent from thecolumn overhead. The impurities, such as, but not intended to be limitedto, p-toluic acid, benzoic acid, 4-carboxybenzaldehyde (4-CBA), and thelike, are recovered from the bottom of the column. In order to make surethe column bottom slurry can be transferred back to the oxidizer, ahigh-boiling diluent is preferably added to the reboiler.

II. Detailed Process Description and Example

The present inventions will be described in terms of the production andrecovery of terephthalic acid (TA) from the air oxidation of p-xylene inthe presence of a solution of components of catalysis in dimethylterephthalate (DMT) or in a benzoic acid-water solvent system. Theoxidizer temperature is preferably between about from 150° C. and about250° C. and the pressure is from between about 5 and about 30 kg percm². Since the oxidizer effluent will contain up to 30% TA, mixing inthe oxidizer is very important in order to maintain the yield andselectivity, and to prevent fouling and blockages. The initial mixing ofthe feed streams may be achieved in a static mixer (outside of theoxidizer). Further mixing may be provided by air sparging and externalcirculation. In the preferred form of the process manganese acetate andcobalt acetate in aqueous solution are fed to the oxidizer to catalyzethe oxidation reactions.

The effluent from the oxidizer at about 160° C. is transferred andfiltered via a first filter to separate the solid from mother liquor(filtrate). During filtering, the solid cake is washed with p-xylenewhich is heated from 30° C. to 100-150° C. The mother liquor istransferred to a first holding tank. The cake washing liquid is removedseparately from the first filter to a second holding tank.

The washed cake is dropped into a first slurry tank to mix with thefollowing streams: (1) NMP or DMAC (selective crystallization solvent)wash liquor (heated from 45 to 100-150° C.); (2) mother liquor (heatedfrom 50 C. to 100-150° C.); and (3) NMP or DMAC (heated from 45° C. to100-150° C.).

The above mixture is then transferred from the bottom of the firstslurry tank to a first dissolver tank. The content in the firstdissolver tank is then heated indirectly from 100-150° C. to 140-200° C.by a hot oil heating coil in the first dissolver tank. About 75% of thep-xylene and 100% of the sparging nitrogen in the mixture is vaporizedfrom the first dissolver tank and removed. Sparging nitrogen is added tothe first dissolver tank to assist the removal of p-xylene. Vaporstreams from the first dissolver tank and a crude crystallizer arecombined into a stream, condensed by a cooler, and sent to a firststorage tank. The bottom effluent from the first dissolver tank istransferred to the crude crystallizer batchwise.

The batch content in the crude crystallizer is reduced in pressure inthe manner described above with concurrent removal of flashed solventand cooled from 140-200° C. to 10-20° C. by an external cooler, togenerate the desired super-saturation for TA crystals to grow. Duringpressure reduction, heat may be added to the batch to effect furthersolvent removal. To improve the crystal size distribution and solidrecovery, crystal seeding may be helpful. At the conclusion of a batchcrystallization cycle, the slurry is dropped into a third holding tankand transferred to a second filter where it is filtered at a continuousrate.

During filtering at the second filter, NMP or DMAC is used to wash thecake in the second filter. The mother liquor plus NMP or DMAC wash arecombined to be fed to a crystallization solvent recovery column. Thewashed cake is dropped into a second dissolver tank where it is mixedwith NMP or DMAC to form the super-saturated feed for a purecrystallizer. NMP or DMAC is heated from 45° C. to 140-200° C. and isfed to the second dissolver tank.

The content of the second dissolver tank is transferred batchwise to thepure crystallizer where the pressure is reduced in the manner describedabove and the temperature is cooled from 140-200° C. to 30-60° C. toinduce TA crystal growth. The cooling is provided by circulating thecrystallizer content through an external cooler. Again, to improve thecrystal size distribution and crystal recovery, crystal seeding may behelpful. At the end of the batch cycle, the slurry is dropped from thepure crystallizer into a feed tank for the third filter.

The slurry is continuously fed to the third filter. The mother liquorfrom the first filter is transferred to a fourth holding tank. The cakeis initially washed with NMP or DMAC at 45° C. to displace the remainingmother liquor from the cake, and then the cake is washed with thelow-boiling displacement solvent, such as water, to displace NMP or DMACfrom the cake or, alternatively, sent to a vacuum dryer. The NMP or DMACwash (from a crystallization solvent storage tank) and the displacementsolvent are then added to the third filter. The NMP or DMAC wash liquidis sent to the first slurry tank, while the displacement solvent istransferred to a fifth holding tank.

The washed cake from the third filter is passed through a wash column ormultistage contactor and counter-current water is added to remove thecrystallization solvent. The slurry from the wash column or contactor isthen fed to the soaker where the temperature is raised to from betweenabout 150-250° C. to remove trapped solvent from the crystals. Theslurry is finally filtered and dropped to a product dryer where water(moisture) in the cake is removed by heating and purging with acounter-current flow of heated nitrogen. The dried PTA product isremoved from the dryer and is stored in the product bin.

The bottom stream from the fifth holding tank (mixture of NMP anddisplacement solvent), together with the liquid from the wash column ormulti-stage contactor, is transferred through a heater (to heat thestream from 25° C. to 80-120° C.) to a displacement solvent evaporator.The displacement solvent vapor from the overhead of the displacementsolvent evaporator is condensed and sent to the displacement solventtank. The bottom stream from the displacement solvent evaporator issplit into two streams: one stream to the vent pot and a second streamto the crystallization solvent tank.

The mother liquor and NMP or DMAC wash from the second filter aretransferred to the crystallization solvent tank and then are fed to theNMP or DMAC recovery column. This stream is heated from 15-25° C. to130-170° C. before entering the recovery column. The overhead vapor iscondensed and sent to a condensate pot. A part of the condensate at160-220° C. is returned to the recovery column as the reflux. The restof the overhead product from recovery column is sent to acrystallization solvent check tank. From the crystallization solventcheck tank, the regenerated NMP or DMAC is pumped to a NMP or DMACstorage tank.

In order to make sure the slurry in the recovery column reboiler can betransferred back to the oxidizer, high-boiling diluent, such as benzoicacid or DMT, is added to the reboiler. The slurry plus the high-boilingdiluent is withdrawn from the bottom of the recovery column and is sentback to the oxidizer.

In FIG. 3, there is shown an arrangement of a crystallizer S-2 usefulfor the practice of the embodiment of the invention in which heat isadded to the crystallizing acid mixture during the times when thepressure is being reduced to flash solvent. As shown in FIG. 3,crystallizer S-2 is there provided with both a cooling recirculationcircuit with exchanger E-8, and a heating recirculation circuit withheater E-8a. Heat is applied to the mixture by heater E-8a duringflashing, and cooling is applied to the mixture at other times byexchanger E-8. Flashed solvent (e.g. NMP or DMAC) is removed throughline 50 for recycling to the recovery column, and the pressure reductionvacuum is also applied to the crystallizer through line 50.

Although a preferred embodiment of the method and apparatus of thepresent invention has been illustrated in the accompanying drawings anddescribed in the foregoing detailed description, it will be understoodthat the invention is not limited to the embodiment disclosed, but iscapable of numerous rearrangements, modifications and substitutionswithout departing from the spirit of the invention as set forth anddefined by the following claims.

What is claimed is:
 1. An apparatus for purifying crude terephthalicacid from a liquid dispersion thereof also containing impuritiesselected from unreacted starting materials, solvents, products of sidereactions and/or other undesired materials comprising:means forfiltering said dispersion to form a crude terephthalic acid filter cake;means for dissolving said filter cake in a selective crystallizationsolvent at an elevated temperature of from between about 140° C. andabout 200° C. to form a solution; means for crystallizing purifiedterephthalic acid from said solution in said crystallization solvent byreducing the temperature and pressure sufficient to flash evaporatesolvent from said terephthalic acid of said solution; and means forseparating said crystallized purified terephthalic acid from saidsolution.
 2. An apparatus for producing purified terephthalic acid fromcrude terephthalic acid comprising:means for dissolving crudeterephthalic acid in a selective crystallization solvent at an elevatedtemperature of from between about 140° C. and about 200° C. to form afirst solution; means for crystallizing first stage purifiedterephthalic acid from said first solution at a reduced temperature andpressure sufficient to flash evaporate solvent from said terephthalicacid; means for separating said crystallized first stage purifiedterephthalic acid from said solution; means for redissolving saidseparated first stage purified terephthalic acid in said selectivecrystallization solvent at an elevated temperature to form a secondsolution; means for crystallizing second stage purified terephthalicacid from said second solution at a reduced temperature and pressuresufficient to flash evaporate solvent from said terephthalic acid; andmeans for separating said crystallized second stage purifiedterephthalic acid from said second solution.
 3. An apparatus forpurifying crude terephthalic acid from a liquid dispersion thereof alsocontaining impurities selected from unreacted starting materials,solvents, products of side reactions and/or other undesired materials,said apparatus comprising:means for dissolving crude terephthalic acidin NMP or DMAC at a temperature of from between about 140° C. and about200° C. to form a first solution; means for crystallizing said dissolvedcrude terephthalic acid from said first solution at a temperature offrom between about 5° C. and about 50° C. and a reduced pressuresufficient to flash evaporate solvent from said terephthalic acid toform first stage purified terephthalic acid; means for separating saidfirst stage purified terephthalic acid from said first solution byfiltration to form a first stage purified terephthalic acid filter cake;means for washing said separated first stage purified terephthalic acidfilter cake with N-methyl pyrrolidone or N,N-dimethyl acetamide; meansfor redissolving said first stage purified terephthalic acid filter cakein NMP at a temperature of from between about 140° C. and about 200° C.to form a second solution; means for crystallizing said redissolvedfirst stage purified terephthalic acid from said second solution at atemperature of from between about 5° C. and about 50° C. and a reducedpressure sufficient to flash evaporate solvent from said terephthalicacid to form second stage purified terephthalic acid; means forseparating said second stage purified terephthalic acid from said secondsolution by filtration to form a second stage purified terephthalic acidfilter cake; means for washing said second stage purified terephthalicacid filter cake with N-methyl pyrrolidone or N,N-dimethyl acetamide;and means for washing said N-methyl Pyrrolidone washed filter cake withwater and means for further treating it by: (a) drying it; or (b) vacuumdrying it, then water soaking it at elevated temperature or contactingit with water in a wash column or multi-stage column, then filtering orcentrifuging it; to provide purified terephthalic acid.